I set up a system to measure the sound pressure level in a location. This consists of a Knowles SPU0410LR5H-QB microphone connected to the codec SGTL5000, which is connected to the micro-controller over I2S.
The SGTL5000 codec microphone input has a 30dB gain before the ADC. I calculate the SPL by sampling the I2S data over 1 second and getting the logarithmic of the RMS value. I took some measurements with the device and a commercial sound level meter and another I2S MEMS microphone using the same calculation method.
Although the measurement method wasn't very scientific, it's clear that there's a deviation from the 45° line indicating that the microphone output level(mV) is not proportional to the sound pressure (Pa).
This is not the result of any Automatic Volume Control by the CODEC by feeding the mic input from a signal generator. (SGTL5000 has AVC but it's turned off in my case).